Method of operating digital-to-analog processing chains, corresponding device, apparatus and computer program product

ABSTRACT

A signal processing chain, such as an audio chain, produces an analog output signal from a digital input signal. The signal processing chain is operated by generating a first flag signal for the analog output signal and one or more second flag signals for the digital input signal. Each flag signal assumes a first level or a second level and is set to the first level when a signal from which the flag is generated has a value within an amplitude window. An amount the first flag signal for the analog output signal and the second flag signal for the digital input signal match each other may be calculated for issuing an alert flag which indicates an impaired operation of the signal processing chain.

FIELD OF THE INVENTION

The description relates to Digital-to-Analog (D/A) processing chains.One or more embodiments may apply to detecting and/or compensatingmalfunctioning of such a signal processing chain. Detecting and/orcompensating an undesired DC offset in a power amplifier of an audiosystem is exemplary of an application of one or more embodiments.

BACKGROUND

Capacitors may be used at the input side of an amplifier to remove anyDC offset in the amplifier input signal, thereby preventing DC currentfrom flowing to the speakers. In the event these capacitors fail or arenot present (e.g., due to an incorrect wiring) substantial damage may becaused to the amplifier and/or the speakers. Audio systems, e.g., forcar audio may include a DC offset detection function based on differentapproaches implemented in the analog domain.

Examples of such approaches are clipping event count or detecting thetime elapsed between consecutive zero crossing events of the analogsignal, that is, between events leading the analog signal to enter/exita certain amplitude window. In the latter case, a vertical displacementof the audio signal as possibly generated by a DC offset may cause thepulses indicative of zero crossing to appear, disappear, move, mixtogether or change length. A DC offset detection circuit sensitive tosuch events may thus detect an offset voltage difference between, e.g.,the output(s) of an audio power amplifier and the audio processorfeeding such amplifier.

In certain implementations, a zero window crossing flag may be used as afeedback signal for a circuit which emulates internally the zerocrossing window and may check, e.g., if the signal at the loudspeakeroutputs of the audio power amplifier crosses the zero level at the sametime as at the output of the audio processor. In certain implementationsaiming at reducing costs, the flags from 4/6 loudspeakers may be mixedtogether (e.g., OR-ed) and then sent to the analog processor. A certainamount of loop delay may be taken into account by a tunable externalcapacitor.

Such implementations rely on dedicated analog components which may befairly bulky and expensive. Also, the amount of DC offset cannot bedetected and no other possible source of impairment taken into account.The detection process may be affected by intrinsic inaccuracy due to theuncertainty of parameters (e.g., the threshold levels, gain, . . . ).

SUMMARY

One or more embodiments may relate to a device and apparatus (e.g., anaudio chain) including such a device as well as to a computer programproduct loadable in the memory of at least one processing device (e.g.,a DSP) and including software code portions for executing the steps ofthe method when the product is run on at least one computer. As usedherein, reference to such a computer program product is understood asbeing equivalent to reference to a computer-readable means containinginstructions for controlling the processing system in order toco-ordinate implementation of the method according to the invention.Reference to at least one processor device is intended to highlight thepossibility for the present invention to be implemented in a modularand/or a distributed form.

One or more embodiments may be based on the recognition that a (fully)digital approach may be used to detect undesired analog signalcomponents since a sampled version of the signal subjected to DACconversion is available at the input of the processing. For instance, inthe case of an audio processing chain fed with a conventional digitalaudio signal (e.g., according to the CD Redbook standard), a sampledversion (44.1/48.0 kHz Fs) of the audio stream may be available at thedigital decoder.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments will now be described, purely by way ofnon-limiting examples, with reference to the annexed figures, wherein:

FIG. 1 is a schematic representation of zero crossing events which maylead a signal to enter/exit a certain amplitude window;

FIG. 2 is general block diagram of a system for checking operation of adigital-to-analog processing chain;

FIG. 3 is an exemplary circuit diagram of a system according to one ormore embodiments;

FIGS. 4 to 7 are schematic representations of how zero crossing eventsof a signal may be affected by various factors;

FIGS. 8 and 9 are block diagrams exemplary of various functions whichmay be performed in one or more embodiments;

FIG. 10, including three portions designated a), b) and c),respectively, is exemplary of possible operation of embodiments;

FIGS. 11 to 13 are block diagrams exemplary of various functions whichmay be performed in one or more embodiments; and

FIG. 14 is a diagram exemplary of possible operation of embodiments.

DETAILED DESCRIPTION

In the ensuing description one or more specific details are illustrated,aimed at providing an in-depth understanding of examples of variousembodiments. The embodiments may be obtained without one or more of thespecific details, or with other methods, components, materials, etc. Inother cases, known structures, materials, or operations are notillustrated or described in detail so that certain aspects ofembodiments will not be obscured.

Reference to an embodiment or one embodiment in the framework of thepresent description is intended to indicate that a particularconfiguration, structure, or characteristic described in relation to theembodiment is comprised in at least one embodiment. Hence, phrases suchas in an embodiment or in one embodiment that may be present in one ormore points of the present description do not necessarily refer to thesame embodiment. Moreover, particular confirmations, structures, orcharacteristics may be combined in any adequate way in one or moreembodiments.

The references used herein are provided merely for convenience and hencedo not define the scope of protection or the scope of the embodiments.

FIG. 1 is a schematic representation of zero crossing events which maylead a signal S (e.g., an analog audio signal) to enter/exit a certainamplitude window ZCW, whose width is identified by a threshold value T.

For the sake of simplicity, throughout this description reference willbe made to a window ZCW centered about a zero value, i.e., a symmetricalwindow between −T and +T. The signal S will thus be inside the windowZCW when having a value between −T and +T, and outside the window ZCWwhen having a value below −T or above +T.

Those of skill in the art will otherwise appreciate that in one or moreembodiments the window ZCW may be non-symmetrical and/or centered abouta non-zero value, so that the crossing events leading the signal S toenter/exit the amplitude window ZCW may not necessarily be zero crossingevents. Consequently, throughout this exemplary description, referringto zero crossing will be for the sake of brevity and simplicity only,with no limitation to the embodiments.

The diagram of FIG. 1 is generally exemplary of the possibility ofgenerating (e.g., a threshold comparator) a flag signal ZCF indicatingif the value of the signal from which the flag is generated S lieswithin the window ZCW or outside the window ZCW.

For instance, as exemplified in FIG. 1, the flag ZCF may be set to 1 ifthe signal S is within the window ZCW and set to 0 if the signal S isoutside the window ZCW. A complementary choice (e.g., 0 if within thewindow ZCW and 1 if outside) is however possible.

The diagram of FIG. 1 is exemplary of the possible effect of a DC offsetleading the signal S to slide down and thus become a signal 5′ whichenters/exits the window ZCW at different times with respect to thesignal S, as reflected by a corresponding change in the flag signalZCF′. The diagram of FIG. 1 is thus exemplary of the possibility ofdetecting a DC offset based on possible changes in the flag signal ZCF.

A number of issues may however arise when attempting to apply such aconcept, e.g., in checking operation of a D/A processing chain (e.g., anaudio amplifier chain fed with an audio signal from a CD player).

The information content of the actual final analog signal resulting fromD/A conversion may in fact be different from the content conveyed by thedigital version, so that the zero crossing behavior of the actual signalderiving from D/A conversion may be different from the behavior of thedigital version of the audio stream.

This may be due to various factors, such as, e.g., the (generallyunknown) delay of the analog signal respect the digital version, due tothe hardware (both analog and digital) between the digital streamprocessing and the power amplifier.

One or more embodiments may rely on the approach schematicallyrepresented in FIG. 1 by producing a sampled version of the flag signalZCF (used as a feedback signal, e.g., from the analog output of an audiopower amplifier) so that the related information may be processed in thedigital domain. In one or more embodiments this may take place viadigital hardware and/or software, thus increasing the level ofmonitoring of the analog environment.

One or more embodiments may thus exploit two types of data, i.e.:

the digital version of the signal before D/A conversion (e.g., an inputdigital audio stream), to be sent to the analog environment; and

a sampled version of the ZCF flag signal as received from the analogenvironment (e.g., from the output of analog power amplifier, forinstance from the wire terminals for connecting to the speaker cables).

One or more embodiments may thus generate in the digital domain (e.g.,44.1 kHz SW) an expected predicted behavior of the (analog) flag ZCF:any differences between the predicted version, e.g., Est. ZCF of theflag and the sampled version of the actual flag HW ZCF from the outputof the D/A processing chain (e.g., from the power audio amplifier) maythus be indicative of a problem occurring, leading to performance beingimpaired.

One or more embodiments may thus rely on the recognition that theeffects of the D/A processing chain on the signal fed thereto may bepredicted/estimated in the digital domain in order to create a goodreplica of the analog waveform.

This concept is schematically represented in the diagram of FIG. 2,where IS denotes a digital signal from a source CD. The signal IS isinput to a physical system 100 which produces therefrom an analog outputsignal OS, e.g., an audio signal to be fed to one or more loudspeakersLS.

At 102 the signal OS is subjected (e.g., via thresholding with athreshold comparator) to ZCF processing as exemplified in FIG. 1 toproduce a ZCF signal, which may be an on/off signal. Also, this on/offZCF signal may be sampled, e.g., with a degree of accuracy comparable tothe accuracy of D/A as performed in the system 100 to generate acorresponding stream of data suitable for processing in the digitaldomain.

For instance, in one or more embodiments, the sampled hardware ZCFsignal HW ZCF may be fed to a performance meter circuit/function 104 tobe compared with a ZCF signal Est. ZCF as estimated (e.g., predicted) bya system model circuit/function 106.

The performance meter 104 may thus compare the two ZCF signals (i.e., HWZCF sampled from the output of the system 100 and Est. ZCF predicted inthe system 106) and, based on the results of comparison, issue an outputsignal (e.g., an alert flag AF) possibly indicative of the performancebehavior of the system 100 swerving from a desired/expected behavior.

In one or more embodiments, as better detailed in the following, thesystem model 106 may operate on the signal IS as well as on inputprovided by a parameter estimation circuit/function 108, which in turnmay operate both on the signal IS and on the sampled ZCF signal obtainedat 102.

Referring to the blocks in FIG. 2 (and in the other figures discussed inthe following) as circuit/function blocks refers to the possibility,already mentioned in the foregoing, that the related processing may beperformed by digital hardware and/or software (e.g., as processingfunctions in a Digital Signal Processor or DSP).

FIG. 3 is an exemplary circuit diagram of a system according to one ormore embodiments where the approach presented in FIG. 2 may be applied.Reference is made again for the sake of simplicity to the exemplary caseof an audio chain receiving an digital input signal IS, e.g., a 44.1kHz/16 bit Redbook audio signal.

Reference 200 in FIG. 3 indicates a digital processor (e.g., a DSP)adapted to process the signal IS and to perform, e.g., the processingtasks exemplified by the blocks 104, 106 and 108 in FIG. 2.

The blocks 202, 204, 206 and 208 are exemplary of a processing chainconfigured to operate on the input signal IS and including both adigital portion D and an analog portion A.

The blocks in question may include,

digital hardware such as an interpolator/error correction code (e.g.,Reed-Solomon) circuitry 202;

a digital-to-analog (DAC) converter stage 204;

analog circuitry such as, e.g., a stereo/multichannelpreamplifier/equalizer 206; and

a power amplifier stage 208 (e.g., multichannel) to produce an analogoutput signal OS for driving a set of loudspeakers LS. DC-offsetfiltering capacitors 208 a may possibly be provided in the poweramplifier stage 208.

The circuit/function 102 of FIG. 2 is exemplified in FIG. 3 as includinga thresholding stage 102 a sensitive to the output signal OS to producea hardware ZCF flag (1 or 0) as discussed in the foregoing.

In one or more embodiments, as exemplified in FIG. 3, the output signalOS may in fact include a plurality of output signal components (e.g.,the output signals of a stereo/multichannel power amplifier). In one ormore embodiments the thresholding stage 102 a may thus include, e.g., aplurality of comparators performing respective windowing functions (withrespective thresholds, which may be the same or different for thevarious signal components) with the flag signal HW ZCF for the analogoutput signal OS produced as a combination, e.g., as produced in an ORgate 102 a′, of the result of thresholding operations performed on aplurality of distinct output signal components.

However produced (e.g., from a single output signal or via a combinationof plural output signal components) the hardware ZCF flag may be sampledin a sampling stage (possibly a sample & hold stage) 102 b with asampling frequency consistent with a degree of accuracy consistent withthe operation frequency of the processor 200 (e.g., 44.1/48 kHz Fs).

In performing a desired A/D conversion, a processing chain asexemplified in FIG. 3 may diversely affect the signals processed thereinby, e.g.:

introducing an analog/digital loop delay, possibly frequency-dependentand different for each power amplifier 208;

producing variable sampling effects at the A/D interface (e.g., S&H 102b);

applying variable analog gains (e.g. at 206 and 208) and thresholdvalues (at 102 a), possibly different for each amplifier/channel; and

introducing a DC offset, possibly different for each poweramplifier/channel.

In one or more embodiments, proper operation of such a processing chainmay be checked based on a comparison of the hardware feedback signal HWZCF which represents the flag ZCF signal generated from the output ofthe chain (e.g., from the output of the power amplifiers 208) as sampledat 102 b with one or more common estimated flags, e.g., Est. ZCF aspredicted by the processor 200.

In one or more embodiments, various conditions may thus apply whencomparing two ZCF flags as schematically illustrated in TABLE 1 below.

TABLE 1 HW ZCF 0 No useful meaning 0 1 False ZCF 1 0 Missed ZCF 1 1Correct ZCF detection

The No ZCF condition, e.g., HW ZCF=0 Est. ZCF =0 (that is, with bothsignals off window) as well as the False ZCF and Missed ZCF conditions(that is, with either signal off window) may be hardly of interest forcomparing two ZCF signals because in those cases the value of the actualoutput analog signal OS can be totally different from any digitalestimation thereof.

In one or more embodiments, only the 1-1 condition, e.g., HW ZCF=1 Est.ZCF=1 (i.e., with both signals lying within the respective thresholdwindow) may be taken into account in comparing the signals IS and OS,e.g., in checking performance of the chain by evaluating the matching oftwo ZCFs.

Stated otherwise, in one or more embodiments, comparison of two ZCFsignals may be elected to be performed only when the two ZCF signals atissue may be regarded as produced from signals whose amplitude fallswithin a respective window ZCW, e.g., when the output of the chain(e.g., from the output(s) of the power amplifiers 208) is within therespective window ZCW (see FIG. 1) and the same may be held to apply forany common estimated signal as predicted in the processor 200.

As indicated, a processing chain as exemplified in FIG. 3 may diverselyaffect the signals processed therein so that the characteristics of,e.g., the HW ZCF flag may be affected. The diagrams of FIGS. 4 to 6 areexemplary of how this may occur.

The diagram of FIG. 4 shows by way of example the possible effect of aloop delay (e.g., due to the elements 202 to 208 of FIG. 3). This may bepossibly frequency-dependent and different, e.g., for each poweramplifier 208, and may produce a corresponding delay (ZCF to ZCF′) inthe HW ZCF flag.

The diagram of FIG. 5 is exemplary of a threshold variation which maycause both width variation and delay (again ZCF to ZCF′) in the HW ZCFflag. It will be appreciated that a thoroughly similar effect may ariseif the threshold(s) set, e.g., at 102 a in FIG. 3 remain(s) constantwhile the gain applied to the signal subject to windowing orthresholding varies (e.g., due to a volume adjustment in an audiochain). Threshold variation will thus be understood herein as a relativevariation of the threshold(s) with respect to the amplitude (gain) ofthe signal to which the threshold(s) apply.

The diagram of FIG. 6 is exemplary of possible effects of the samplingprocess, e.g., a variation of the sampling instants for an otherwiseconstant sampling rate: a flag ZCF notionally having the time behaviorillustrated as a solid line may be sampled as a time-offset flag ZCF′ ofa different width if the sampling instants correspond to the circles,and even as a no-flag ZCF signal if the sampling instants correspond tothe triangles.

The diagram of FIG. 7 is exemplary of the possible effect of DC offseton a sampling process at the instants indicated by circles. Even withthe same sampling instants, a flag ZCF notionally having the timebehavior illustrated as a solid line may be sampled either as atime-offset (delayed) flag ZCF′ or as a time-offset (anticipated) flagZCF″.

Those skilled in the art will appreciate that the discussion aboveapplies irrespective of whether a 1 or 0 is assigned to the ZCF flag toindicate that the corresponding signal lies within the window ZCW.

In one or more embodiments, processing of the digital input data stream(e.g., 44.1 kHz) at 200 may be tuned by acting on the estimationparameters, in order to match the analog output signal when the D/Aprocessing chain (e.g., 202, 204, 206, 208) is operating correctly, sothat a possible mismatch may be reliably taken as an indication ofoperation being no longer satisfactory.

One or more embodiments may thus involve comparing (e.g., at 104 in FIG.2) two flag signals HW ZCF (obtained from the analog output) and Est.ZCF (obtained from the digital input) and calculating as a function ofthe result of the ZCF comparison a performance check signal whichindicates an amount the respective flag signals HW ZCF, Est. ZCFcorrespond (i.e., match) with each other.

The performance check signal in question may thus indicate theperformance quality of the processing chain (202, 204, 206, 208) and maytake, e.g., the form of an alert flag AF issued from the performancemeter block 104 when the degree of matching of the flag signals HW ZCF,Est. ZCF falls below a certain performance quality threshold. That is,when the two flag signals HW ZCF, Est. ZCF are found no longer to matchsatisfactorily with each other.

In one or more embodiments, a system, such as, e.g., an audio chain asexemplified herein, may be pre-loaded (e.g., in production) with valuesof estimation parameters applied, e.g., to a system model block 106 asschematically represented in FIG. 8. For instance, the digital inputsignal IS may be subjected to one or more of, e.g., a bulk integer delay1060, adding/subtracting a DC offset at 1062, a further fractional delay1064 and a windowing (thresholding) operation in order to produce anestimated ZCF value Est. ZCF which, when the system operates asexpected, matches (e.g., within a certain tolerance range) the flagsignal HW ZCF obtained for the analog output signal OS, e.g., from the S& H block 102 b.

A possible mismatch of the flag signals HW ZCF, Est. ZCF may thus beheld to indicate performance degradation (e.g. malfunctioning) so that acorresponding alert flag AF may be issued (e.g., from the block 104 ofFIG. 2), e.g., as a warning signal (e.g., optical and/or sound).

In one or more embodiments as discussed in the following in connectionwith FIGS. 9 to 14, a system as the audio chain exemplified herein maybe configured for possible estimation, performed run-time, e.g., duringoperation of operating parameters which may be fed e.g. to the systemmodel block 106 from the parameter estimation block 108. This may bearranged as exemplifies in FIG. 9. For example, this may be in view of apossible operation based on the concept outlined in FIG. 10, with theestimated parameters converging to a correct value EV.

In one or more embodiments, a count of matches of flag signals includinga hardware flag signal (e.g., HW ZCF) obtained from the analog output OSwith certain flag signals obtained by processing the digital inputsignal IS may be used as a metrics in such estimation process which issuitable to be performed run time, e.g., during system operation.

In one or more embodiments, one or more such estimated parameters (e.g.,DC offset, delay) as considered herein may be used to generate an alertsignal (e.g., AF as described previously).

In one or more embodiments (e.g., as further discussed in the final partof this description), one or more such estimated parameters (e.g., DCoffset, delay) as considered herein may be used to drive a control loopin order to compensate the phenomenon leading to impairment ofperformance.

In these cases, the system will be able to self-compensate the source ofimpairment thus ensuring that, e.g., HW ZCF and Est. ZCF as discussedpreviously will mostly match, so that direct comparison of these twoZCFs will no longer be essential.

Similar comparisons between, e.g., HW ZCF and other ZCF flags obtainedfrom the digital input signal IS may however be performed in parameterestimation/tracking as performed in the exemplary estimation layoutshown in FIG. 9, where the block 108 includes, possibly in addition toan adjustable integer or bulk delay ADJ (adapted to be used for e.g.trimming the estimator):

a first estimation circuit/function 1082 for a fractional (e.g., fineadjustment) delay to be applied at 1064;

second estimation circuit/function 1084 of one or more threshold(s) forwindowing at 1066; and

a third estimation circuit/function 1086 for a DC offset to be appliedat 1062.

In one or more embodiments as exemplified herein, the bulk or integerdelay to be applied at 1080 may be an initial, rough adjustmentparameter ADJ of the system, not intended to be subsequently modified.

One or more embodiments may provide for a different choice of theparameters being estimated: for instance, in one or more embodiments,not all of delay, threshold and DC offset but only one or two of thereparameters may be considered. In one or more embodiments, otherparameters may be added to and/or substituted for delay, threshold andDC offset.

The diagrams of FIGS. 10 to 14 are exemplary of one or more embodimentswhere the estimation parameters (e.g., block 108 in FIG. 2) may bedetermined during operation of the system (e.g., during a training phaseand/or during regular operation) so that events resulting in impairedsystem performance may be detected (and possibly made known, e.g., byissuing an alert flag AF) and/or compensated.

In one or more embodiments as exemplified in FIG. 9, the estimatorblocks 1082, 1084 and 1086 may operate by determining an exact (target)value EV for each parameter as a stationary point for a performancefunction.

Such an approach is schematically represented in the diagrams of FIG.10, where:

the portion a) shows a current estimated value CE which is, e.g., lowerthan the exact value EV, so that a corresponding performance functionexhibits a positive gradient PG towards a peak (e.g., optimum) value;

the portion b) shows a current estimated value CE which is, e.g., higherthan the exact value EV, so that the corresponding performance functionexhibits a negative gradient NG away from the peak value; and

the portion c) shows a current estimated value CE which is at or inclose proximity of the exact value EV, so that the correspondingperformance function no longer exhibits a gradient, i.e., a zero (oralmost zero) gradient GRAD=0 which indicates that the current estimatedvalue CE has reached the exact value EV.

In one or more embodiments, specific performance functions may bedefined for each parameter. For instance, a useful performance functionboth for fractional delay and for DC offset may be selected as thedifference of successful detections (1-1: see the last line in thepreceding table) of certain ZCF flags discussed in the following over afixed time frame, e.g., based on a so-called Integrate and Dump (I&D)approach.

In one or more embodiments, a useful performance function forthreshold(s) may be selected as the difference of false detection (1-0)events vs. missed detection (0-1) events of certain ZCF flags discussedin the following (see the second and third lines in the precedingtable), again over a fixed time frame, e.g., based on an I&D approach.

In one or more embodiments, the performance function (selected, e.g., asexemplified previously) may be evaluated at two nearby points, and theestimated value of the parameter may be tuned with a control loop. Inone or more embodiments such a loop may include an integral operator.

In one or more embodiments, the estimation process converges to thecorrect values for all the parameters, while an estimation for eachparameter may become available to the user as a side effect.

In one or more embodiments, the estimation process may converge to thecorrect values even if fed with a hardware flag HW flag obtained as acombination (e.g., an OR-ed configuration as exemplified at 102 a inFIG. 3) of the individual flags from different signal components, e.g.,from different amplifiers 208 in a chain as exemplified in FIG. 3.

The whole of this processing lends itself to being effected in digitalform, e.g., by a conventional DSP, e.g., as exemplified in thefollowing.

The diagram of FIG. 11 is representative of an exemplary arrangement fora fractional delay estimator 1082 to provide an estimated fractionaldelay value to the delay element (filter) 1064 of FIG. 8.

In one or more embodiments, input to the estimator 1082 may include:

the input data stream, after application of the integer or bulk delay at1060, denoted IS′;

a DC offset value as estimated by the estimator 1086;

a threshold value T as estimated by the estimator 1084; and

the hardware ZCF flag, HW ZCF, from the block 102.

In one or more embodiments, the data stream IS′ and the DC offset valueare added in a node 300 and fed to two fractional delay blocks 302 a,302 b to which the current fractional delay estimation (that is, CE inthe diagrams of FIG. 10) is fed with added positive and negative deltas+Δ and −Δ, respectively. The fractional delay blocks 302 a, 302 b maythus apply two different values of fractional delay so that a sort ofcorresponding gradient may be computed: see, e.g., PG or NG in FIG. 10,parts a) and b).

The outputs from the blocks 302 a, 302 b are subjected to windowing inzero cross blocks 304 a, 304 b (as a function of the threshold T) andthe ZCFs, e.g., ZCF′_(FD) and ZCF″_(FD), obtained thereby may fed tological product (AND) blocks 306 a, 306 b whose other inputs are fedwith the signal HW ZCF and whose outputs converge to a difference node308 and on to an integrate-and-dump (I&D) block 310.

Such processing produces a difference of successful detections(essentially like 1-1 events in the previous table) over a fixed timeframe, which may be used as a performance function as discussedpreviously.

Possible low pass filtering at 312 derives from the result of I & Dprocessing at 310 a fractional delay estimation error, for use by anintegrator block 314. The output from the integrator 314 becomingconstant, see, e.g., GRAD=0 in FIG. 10, part c), will correspond to acorrect estimated value EV for the fractional delay, e.g., for feedingto the fractional delay filter 1064 of FIG. 8 (and possibly to otherprocessing blocks as otherwise detailed).

The output from the block 314 is also fed to two delta generation blocks316 a, 316 b for feeding to the fractional delay blocks 302 a, 302 b asdiscussed previously.

The diagram of FIG. 12 is representative of an exemplary arrangement fora DC offset estimator 1086 to provide an estimated DC offset value tothe summation node 1062 of FIG. 8.

In one or more embodiments, input to the estimator 1086 may include:

the input data stream, again after application of the integer or bulkdelay at 1060, denoted IS′;

a fractional delay value as estimated by the estimator 1082 discussedpreviously;

a threshold value T as estimated by the estimator 1084; and

the hardware ZCF flag, HW ZCF, from the block 102.

In one or more embodiments, the data stream IS′ and the fractional delayvalue are combined in a fractional delay block 400 wherein the latest(i.e. most recent) estimate available for the fractional delay isapplied to the data stream IS′ to be fed to two summation nodes 402 a,402 b to which the current DC offset estimation (that is, CE in thediagrams of FIG. 10) is fed with added negative and positive deltas −Δand +Δ, respectively, so that, again a sort of corresponding gradientmay be computed: see again, e.g., PG or NG in FIG. 10, parts a) and b).

The outputs from the nodes 402 a, 402 b are subjected to windowing inzero cross blocks 404 a, 404 b (as a function of the threshold T) andthe ZCFs obtained thereby, e.g., ZCF′_(DC) and ZCF″_(DC), are fed tological product (AND) blocks 406 a, 406 b whose other inputs are fedwith the signal HW ZCF and whose outputs converge to a difference node408 and on to an integrate-and-dump (I&D) block 410.

Such processing produces the difference of successful detections (againessentially like 1-1 events in the previous table) over a fixed timeframe which may be used as a performance function as discussedpreviously.

Possible low pass filtering at 412 derives from the result of I & Dprocessing at 410 a DC offset estimation error, for use in an integratorblock 414. As is the case of the integrator block 314 discussedpreviously, the output from the integrator 414 becoming constant—seeagain, e.g., GRAD=0 in FIG. 10, part c)—will correspond to a correctestimated value EV for the DC offset for feeding, e.g., to the DC offsetsummation node 1062 of FIG. 8 (and possibly to other processing blocksas otherwise detailed). The output from the block 414 is also fed to twodelta generation blocks 416 a, 416 b for feeding to the summation nodes402 a, 402 b as discussed previously.

Finally, the diagram of FIG. 13 is representative of an exemplaryarrangement for a threshold estimator 1084 to provide at least oneestimated threshold value to the windowing logic 1066 of FIG. 8. Whileplural thresholds may be estimated, e.g., in the case of a non-symmetricwindow ZCW, a single threshold value T (e.g., for producing asymmetrical −T, +T window) is considered here for the sake ofsimplicity.

In one or more embodiments, input to the estimator 1084 may include:

the input data stream IS;

a fractional delay value as estimated by the estimator 1082 discussedpreviously;

a DC offset value as estimated by the estimator 1086; and

the hardware ZCF flag, HW ZCF, from the block 102.

In one or more embodiments, the data stream IS and the fractional delayvalue are combined in a fractional delay block 500 wherein (as is thecase for the fractional delay block 400 discussed previously) the latest(i.e., most recent) estimate available for the fractional delay isapplied to the data stream IS to be fed to a summation node 502 to whichthe current DC offset estimation (that is, CE in the diagrams of FIG.10) is fed.

The output from the node 502 are subjected to windowing in zero crossblocks 504 a, 504 b as a function of the current estimation threshold Twith added positive and negative deltas +Δ and −Δ, respectively so that,once more, a sort of corresponding gradient may be computed: see oncemore, e.g., PG or NG in FIG. 10, parts a) and b).

The ZCFs obtained thereby, e.g., ZCF′_(T) and ZCF″_(T), are fed, afterlogical inversion (that is, in negated form) to logical product (AND)blocks 506 a, 506 b whose other inputs are fed with the signal HW ZCFand whose outputs converge to a difference node 508 and on to anintegrate-and-dump (I&D) block 510.

Such processing produce the difference of false detections (essentiallylike 0-1 events in the previous table) vs. missed detections(essentially like 1-0 events in the previous table) over a fixed timeframe which may be used as a performance function as discussedpreviously.

This approach is further exemplified in FIG. 14. There an exactthreshold ET is shown in the upper diagram of portion a), while acorresponding ZCF for a signal S is shown in full line in portion c).

The effect of an increase (+Δ) and a decrease (−Δ) in the threshold(again a symmetrical threshold centered about zero is assumed for thesake of simplicity) in terms of missed detections MD and falsedetections FD is exemplified in portions b) and d) of the diagram.

Possible low pass filtering at 512 derives from the result of I & Dprocessing at 510 a threshold estimation error, for use in an integratorblock 514. As is the case of the integrator blocks 314 and 414 discussedpreviously, the output from the integrator 514 becoming constant—seeonce more, e.g., GRAD=0 in FIG. 10, part c)—will correspond to a correctestimated value EV for feeding, e.g., to the windowing logic 1066 ofFIG. 8 (and possibly to other processing blocks as otherwise detailed).

The output from the block 514 is also fed to two delta generation blocks516 a, 516 b for feeding to the zero cross blocks 504 a, 504 b asdiscussed previously. A basic arrangement as disclosed herein lendsitself to a wide variety of possible modes of operation. For instance,in one or more embodiments, computing power may be saved by splittingin, e.g., two steps processing as discussed above.

This may involve measuring the parameters of interest during theproduction phase (e.g., in the factory, possibly by using optimizedsignals for system training) and assume that these may remainessentially constant over time as long as the processing chain beingchecked operates correctly. These values for the parameters may be inputas adjustment parameters to the system model block (see, e.g., 106 inFIG. 2) so that during the system lifetime an alert flag AF will beemitted (only) if performance shows a degradation. In that way, runtimeestimation of the parameters will not be performed, by reducing theprocessing requirements accordingly.

In one or more embodiments, the two-phase approach considered in theforegoing may reduce computational complexity, while permitting ZCFcorrelation (e.g., matching) during audio streaming, with parameters andchannels not distinct and training time absorbing time in theproductions process.

Specific application options/requirements may dictate whether such anapproach may be preferred over continuous tracking of the parameters,e.g., a tracking process running in background during audio streaming,which may offer the advantage of all parameters being under control,possibly with distinct channels.

In one or more embodiments, continuous tracking of one or more operatingparameters may permit prompt detection of any parameter falling out of apredefined acceptable range (e.g., DC offset) so that a correspondingalert flag AF may be emitted to indicate performance degradation even inthe absence of any comparison of HW ZCF and Est. ZCF, while in one ormore embodiments where continuous tracking is not implemented, checkingof performance parameters may be based on HW ZCF and Est. ZCF only,e.g., by determining that the HW ZCF derived from the output of the D/Aprocessing chain, e.g., from the output(s) of the power amplifiers 208of FIG. 23, differs (e.g., does not match with) the Est. ZCF predictedfor the digital input signal IS, e.g., in block 106 of FIG. 2.

One or more embodiments may thus be based on the recognition that:

one or more ZCF flags (such as Est. ZCF; ZCF′_(FD) and ZCF″_(FD);ZCF′_(DC) and ZCF″_(DC); ZCF′_(T) and ZCF″_(T),) may be generated forthe digital input signal by being estimated in digital form—e.g., byapplying to the digital input signal IS e.g. a time delay (bulk orfractional), a DC offset and a windowing based on a giventhreshold—without D/A conversion of the digital input signal IS;

although generated in an analog context, the HW ZCF derived from theoutput of the D/A processing chain is intrinsically a binary signal(either 1 or 0). Once sampled, e.g., at the same sampling frequency thedigital input signal, the HW ZCF is a digital signal which lends itselfeasily to be compared with any of the ZCF flags (e.g., Est. ZCF;ZCF′_(FD) and ZCF″_(FD); ZCF′_(DC) and ZCF″_(DC); ZCF′_(T) and ZCF″_(T))generated for the digital input signal.

By way of a non-limiting example, possible options for comparing HW ZCFwith any ZCF flag (e.g., Est. ZCF) generated for the digital inputsignal may include counting the number of matches in a definedtimeframe, calculating the ratio of the number of matches to the totalnumber of HW ZCF flags over a certain time frame, calculating thevariance of the number of success matching over a certain time frame, orany other ways of determining the extent or degree the two ZCF flags,correspond (e.g., coincide) with each other.

In one or more embodiments, in addition or as an alternative togenerating an alert flag AF indicative of, e.g., malfunctioning,performance check processing as exemplified in the foregoing may beexploited (e.g., by processing in the block 200) to compensate/remedyfor unsatisfactory performance.

In one or more embodiments, such processing may include a control loop,such as, e.g., an integrator whose input may be, for instance, aparameter such as the estimated DC offset from the DC offset estimator1086 of FIG. 12 and whose output is a value to be subtracted from/addedto the input data stream (e.g., the digital input signal IS, suchprocessing occurring, e.g., in the DSP 200 of FIG. 3). Convergence ofsuch a control loop will result in a DC offset measured to bepractically zero and the integrator output the DC value to becompensated.

One or more embodiments may implement such a control function in realtime, while the system (e.g., an audio chain) processes the real (e.g.,audio) signal and the integrator tracks, e.g., the DC offset variations.

In one or more embodiments, the DC offset value may be calculated to bethen subtracted (e.g., in an open loop arrangement) from the input datastream. In such an embodiment, the DC offset will not be tracked, butthe unknown changes in its value may be monitored in an indirect mannervia the matching function discussed previously.

In one or more embodiments, the DC offset being compensated digitally asdiscussed previously, will make it possible to dispense with any DCdecoupling capacitors, leading to an appreciable HW optimization, sincethe DC offset measured value will be constantly maintained to zero,e.g., by a simple PID controller implementing the related control loop.

In one or more embodiments, an accurate, closed loop, real time measureof the DC offset will then be available. In one or more embodiments, asimilar approach can be adopted also for any of the other parametersdiscussed in the foregoing (delay, thresholds, and so on).

More generally, various different modes of operation may be adopted fora system (digital-to--analog processing chain) as exemplified in theforegoing. For instance, one or more embodiments may involve an initialtraining for the identification of the parameters; these estimatedparameters may then be applied in a static manner to the system model(e.g., block 106 of FIG. 2). That is, the model of the system will notchange over time and a possible mismatch (lack of correspondence) of HWZCF and Est. ZCF as measured, e.g., on the basis of any of the metricsexemplified in the foregoing may be construed as indicating an undesiredalteration of the electrical parameters (e.g., a DC offset) of thesystem. Based thereon an alert flag AF may be raised to advise, e.g.,the user of such an undesired mode of operation.

In this operation mode, the two flag signals HW ZCF and Est. ZCF may beused for the initial parameter estimation and for detecting an alertcondition.

One or more embodiments may involve an operation mode wherein, after astart-up phase where a mismatch between HW ZCF and Est. ZCF willexpectedly be detected (i.e., with lack of correspondence between them),the system will implement a tracking scheme with the estimationparameters updated run-time by an estimation loop and applied to thesystem, e.g., via the processing involving the ZCF flags ZCF′_(FD) andZCF″_(FD); ZCF′_(DC) and ZCF″_(DC); ZCF′_(T) and ZCF″_(T) discussed inconnection with FIGS. 10 to 14.

Once convergence is achieved, HW ZCF and Est. ZCF will expectedly besimilar, that is essentially matching, insofar as the parameterestimates are valid. In that case rather than on the HW ZCF v. Est. ZCFmatching, an alert flag AF may be issued based, e.g., on the very valuesof the estimated parameter(s), e.g., because the estimated value of acertain parameter lies outside a certain range acceptable for operation.

Any possible mismatch (lack of correspondence) of the two ZCF signalsmay indicate that the estimated values are not reliable because, e.g.,the estimation process has not converged yet or because is has beensomehow altered, with the possibility that it may re-converge more orless rapidly. Any changes in the physical changes of the system (again aDC offset may be case in point) are however likely to occur slowly sothat they may be effectively tracked by the tracking loop. For thatreason, an alert flag AF may be issued based, e.g., on the very valuesof the estimated parameter(s).

One or more embodiments as exemplified in the final portion of theprevious description may involve a control loop adapted to affect thedigital data stream IS input to the system. In that case, the estimatedvalue of the parameter(s) may not be just monitored in order tounderstand what is going on, but may be exploited for pre-compensatingthe digital input IS in order to cancel out an undesired final effect onthe output signal. For instance, a digital DC offset may be added to orsubtracted from the digital input signal IS by adding to/subtractingfrom the sample data a certain set of bits corresponding to the offsetadded/subtracted in order to compensate (i.e., cancel out) an undesiredDC offset applied to loudspeaker LS.

Still for instance, a digital delay may be applied to the digital inputsignal IS in order to compensate (i.e., cancel out) a different delay onstereo channels. In that case the system will continue to operate evenin the presence of a possible source of malfunctioning (by compensatingit), with the effect of a possible source of malfunctioning minimizeddue to (pre)compensation of the input signal.

In one or more embodiments, various analog components may thus bedispensed with, with advantages in terms of cost and in space, by movingtowards a fully digital approach, where SW implementation is possible,while permitting real time measurement of relevant parameters

One or more embodiments may thus permit to operate a signal processingchain (e.g., 202, 204, 206, 208 in FIG. 3) which produces an analogoutput signal OS from a digital input signal IS.

In one or more embodiments at least one first flag signal (e.g., HW ZCF)is generated for the analog output signal OS and one or more second flagsignals (e.g. Est. ZCF; ZCF′_(FD), ZCF″_(FD): ZCF′_(DC), ZCF″_(DC);ZCF′_(T), ZCF″_(T)) are generated for the digital input signal IS.

In one or more embodiments, each of these flag signals assumes a firstlevel and a second level (e.g., 1 and 0) and is set to the first level(e.g., 1) when a signal from which the flag is generated has a valuewithin a certain amplitude window. For instance, in case a single outputsignal OS is present, the flag HW ZCW may be 1 if the output signal OSlies within its thresholding window and 0 otherwise.

In the exemplary case represented in FIG. 3 herein, the various analogoutput signal components generate, due to thresholding at 102 a,respective on/off signals which, due to the OR function performed at 102a′, may produce a flag HW ZCW for the output signal OS which may be 1 ifany the analog output signal components subject to thresholding at 102 alies within its thresholding window.

The same applies to any of the second flag signals such as Est. ZCF;ZCF′_(FD), ZCF″_(FD); ZCF″_(FD), ZCF″_(DC); ZCF′_(T), ZCF″_(T), with:

Est. ZCF generated from the signal output of the fractional delay filter1064 via thresholding at 1066 in FIG. 8;

ZCF′_(FD), ZCF″_(FD) generated from the output signals of the fractionaldelay blocks 302 a, 302 b via thresholding at 304 a, 304 b in FIG. 11;

ZCF′_(DC), ZCF″_(DC) generated from the output signals of the summationnodes 402 a, 402 b via thresholding at 404 a, 404 b in FIG. 12; and

ZCF′_(T), ZCF″_(T) generated from the output signal of the summationnode 502 via thresholding at 504 a, 504 b in FIG. 13.

One or more embodiments as exemplified herein, the first flag signal HWZCF and one or more of the second flag signals Est. ZCF; ZCF′_(FD),ZCF″_(FD); ZCF′_(DC), ZCF″_(DC); ZCF′_(T), ZCF″_(T) may be exploited invarious ways in operating a digital-to-analog signal processing chain asexemplified herein. To that effect, in one or more embodiments, a firstflag signal HW ZCF for the analog output signal OS may be sampled at thesame sampling frequency of the digital input signal IS.

For instance, in one or more embodiments, the first flag signal HW ZCFobtained from the analog output OS and the second flag signal Est. ZCFobtained from the digital input signal IS may be compared (e.g., at 104in FIG. 2) to calculate an amount the first flag signal HW ZCF and thesecond flag signal Est. ZCF match with each other.

In one or more embodiments, the amount may be selected out of:

the count of matches of the first flag signal HW ZCF and the second flagsignal Est. ZCF over a certain time frame;

the ratio of the number of matches of the first flag signal HW ZCF andthe second flag signal Est. ZCF to the total number of either type (HWZCF resp. Est. ZCF) of flag signals over a certain time frame; and

the variance of the number of matches of the first flag signal HW ZCFand the second flag signal Est. ZCF over a certain time frame.

In one or more embodiments, e.g., in those embodiments where theprocessing chain is pre-loaded with estimation parameter for Est. ZCFwhich are no longer modified during operation, a performance checksignal AF may be generated indicative of the amount the first flagsignal HW ZCF for the analog output signal OS and the second flag signalEst. ZCF the digital input signal IS match with each other. Suchperformance check signal may include an alert flag AF issued when thatamount of matching falls below a certain performance quality threshold.

In addition or as an alternative to the foregoing, in one or moreembodiments, a first flag signal HW ZCF obtained from the analog outputOS and the various second flag signals ZCF; ZCF′_(FD), ZCF″_(FD);ZCF′_(DC), ZCF″_(DC); ZCF′_(T), ZCF″ obtained in various ways from thedigital input signal IS as exemplified in connection with FIGS. 10 to 14may be processed to calculate (e.g., in blocks 1082, 1084, 1086 in FIG.9) an estimate of at least one operating parameter (e.g. FD, DC offset,T) of the signal processing chain 202, 204, 206, 208. As exemplified inconnection with FIG. 10, the estimate of at least one operatingparameter (FD, DC offset, T) of the signal processing chain 202, 204,206, 208 may be calculated as a target value EV which optimizes arespective performance function, preferably by minimizing the gradientthereof (see, e.g., GRAD=0 in FIG. 10).

In one or more embodiments, if the estimated operating parameterincludes a delay FD and/or a DC offset, the performance function may beselected as the difference (e.g., computed at nodes 308, 408 in FIGS. 11and 12) of successful detections (1-1 in the previous table) of thefirst flag signal HW ZCF the second flag signal (see e.g. ZCF′_(FD),ZCF″_(FD) for the fractional delay in FIG. 11 or ZCF′_(DC), ZCF″_(DC)for the DC offset in FIG. 12) over a time frame, as determined e.g. bythe Integrate & Dump blocks 310, 410.

In one or more embodiments, if the estimated operating parameterincludes a windowing threshold T, the performance function may beselected as the difference (e.g., computed at node 508 in FIG. 13) offalse detection events and missed detection events (see, e.g., FD and MDin FIG. 14) of the first flag signal HW ZCF and the second flag signalZCF′_(T), ZCF″_(T) over a time frame, as determined, e.g., by theIntegrate & Dump block 510 in FIG. 13.

In one or more embodiments, a performance check signal such as an alertflag AF may be issued if the estimated value of at least one operatingparameter such as, e.g., FD, DC offset or T lies outside a certain rangeacceptable for operation.

In one or more embodiments, wherein an estimate is calculated of atleast one operating parameter (e.g., FD, DC offset, T) of the signalprocessing chain 202, 204, 206, 208, the estimated operating parametersuch as, e.g., DC offset may be fed to a control loop, preferably actingon the digital input signal IS, to compensate for system performanceimpairment, e.g., as possibly induced by the estimates operatingparameter moving outside a certain range acceptable for operation.

In one or more embodiments, generating the one or more second flagsignals for the digital input signal IS may include applying to thedigital input signal IS one or more of:

a delay, as is the case of the delays 1060 and 1064 applied to IS togenerate Est. ZCF in FIG. 8, or the delays ADJ, 302 a, 302 b; ADJ, 400;500 of FIGS. 9 to 13,

a DC offset, as is the case of the DC offset 1062 applied to IS togenerate Est. ZCF in FIG. 8, or the DC offsets 300, 302 b; 402 a, 402 b;502 of FIGS. 9 to 13; and

windowing as a function of one or more thresholds, as is the case of thewindowing 1066 applied to IS to generate Est. ZCF in FIG. 8, or thewindowing blocks 304 a, 304 b; 404 a, 404 b; 504 a, 504 b of FIGS. 9 to13.

Without prejudice to the underlying principles, the details andembodiments may vary, even significantly, with respect to what isillustrated herein purely by way of non-limiting examples, withoutthereby departing from the extent of protection. The extent ofprotection is determined by the claims that follow.

1-13. (canceled)
 14. A method of operating a signal processing chainproducing an analog output signal from a digital input signal, themethod comprising: generating a least one first flag signal for theanalog output signal and at least one second flag signal for the digitalinput signal, with the at least one first and second flag signals havinga first level and a second level and each being set to the first levelwhen a respective signal from which the at least one first and secondflags is generated has a value within an amplitude window; andcalculating, as a function of the at least one first flag signal and theat least one second flag signal, at least one of an amount at least onefirst flag signal for the analog output signal and at least one secondflag signal for the digital input signal match each other, and anestimate of at least one operating parameter of the signal processingchain.
 15. The method of claim 14, further comprising sampling eachfirst flag signal for the analog output signal at a same samplingfrequency of the digital input signal.
 16. The method of claim 14,further comprising generating a second flag signal for the analog outputsignal for a combination of a plurality of output signal components,with the combination being an OR combination.
 17. The method of claim14, further comprising calculating the amount the at least one firstflag signal for the digital input signal and the at least one secondflag signal match each other as at least one of the following: a countof matches of the at least one first flag signal and the at least onesecond flag signal over a time frame; a ratio of a number of matches ofthe at least one first flag signal and the at least one second flagsignal to a total number of either type of the flag signals over thetime frame; and a variance of a number of matches of the at least onefirst flag signal and the at least one second flag signal over the timeframe.
 18. The method of claim 14, further comprising: calculating anamount the at least one first flag signal for the digital input signaland the at least one second flag signal match with each other; andgenerating a performance check signal indicative of the amount, with theperformance check signal comprising an alert flag issued when the amountfalls below a performance quality threshold.
 19. The method of claim 14,further comprising calculating the estimate of at least one operatingparameter of the signal processing chain as a target value based on arespective performance function by reducing a gradient of the respectiveperformance function.
 20. The method of claim 19, wherein the at leastone estimated operating parameter comprises at least one of a delay anda DC offset, and selecting the performance function as a difference ofsuccessful detections of the at least one first flag signal and the atleast one second flag signal over a time frame.
 21. The method of claim19, wherein the at least one estimated operating parameter includes awindowing threshold, and selecting the performance function as adifference of false detection events and missed detection events of theat least one first flag signal and the at least one second flag signalover a time frame.
 22. The method of claim 14, further comprisinggenerating a performance check signal if the estimate of the at leastone operating parameter lies outside a range for operation, with theperformance check signal including an alert flag.
 23. The method ofclaim 14, further comprising feeding the estimate of the at least oneoperating parameter to a control loop acting on the digital input signalto compensate for system performance impairment.
 24. The method of claim14, wherein generating the at least one second flag signal for thedigital input signal comprises applying to the digital input signal atleast one of a delay, a DC offset, and an amplitude window.
 25. A devicefor operating a signal processing chain producing an analog outputsignal from a digital input signal, the device comprising: at least oneflag generating circuit configured to generate a least one first flagsignal for the analog output signal and at least one second flag signalfor the digital input signal, with the at least one first and secondflag signals having a first level and a second level and each being setto the first level when a respective signal from which the at least onefirst and second flags is generated has a value within an amplitudewindow; and a processor configured to calculate, as a function of the atleast one first flag signal and the at least one second flag signal, atleast one of an amount at least one first flag signal for the analogoutput signal and at least one second flag signal for the digital inputsignal match each other, and an estimate of at least one operatingparameter of the signal processing chain.
 26. The device of claim 25,wherein said processor is configured to sample each first flag signalfor the analog output signal at a same sampling frequency of the digitalinput signal.
 27. The method of claim 25, wherein said at least one flaggenerating circuit is configured to generate a second flag signal forthe analog output signal for a combination of a plurality of outputsignal components, with the combination being an OR combination.
 28. Themethod of claim 25, wherein said processor is configured to calculatethe amount the at least one first flag signal for the digital inputsignal and the at least one second flag signal match each other as atleast one of the following: a count of matches of the at least one firstflag signal and the at least one second flag signal over a time frame; aratio of a number of matches of the at least one first flag signal andthe at least one second flag signal to a total number of either type ofthe flag signals over the time frame; and a variance of a number ofmatches of the at least one first flag signal and the at least onesecond flag signal over the time frame.
 29. The method of claim 25,wherein said processor is configured to: calculate an amount the atleast one first flag signal for the digital input signal and the atleast one second flag signal match with each other; and generate aperformance check signal indicative of the amount, with the performancecheck signal comprising an alert flag issued when the amount falls belowa performance quality threshold.
 30. The method of claim 25, whereinsaid processor is configured to calculate the estimate of at least oneoperating parameter of the signal processing chain as a target valuebased on a respective performance function by reducing a gradient of therespective performance function.
 31. An apparatus comprising: a signalprocessing chain configured to drive at least one loudspeaker, and toproduce an analog output signal from a digital input signal; and adevice configured to operate said signal processing chain and comprisingat least one flag generating circuit configured to generate a least onefirst flag signal for the analog output signal and at least one secondflag signal for the digital input signal, with the at least one firstand second flag signals having a first level and a second level and eachbeing set to the first level when a respective signal from which the atleast one first and second flags is generated has a value within anamplitude window, and a processor configured to calculate, as a functionof the at least one first flag signal and the at least one second flagsignal, at least one of an amount at least one first flag signal for theanalog output signal and at least one second flag signal for the digitalinput signal match each other, and an estimate of at least one operatingparameter of the signal processing chain.
 32. The apparatus of claim 31,wherein said processor is configured to sample each first flag signalfor the analog output signal at a same sampling frequency of the digitalinput signal.
 33. The apparatus of claim 31, wherein said at least oneflag generating circuit is configured to generate a second flag signalfor the analog output signal for a combination of a plurality of outputsignal components, with the combination being an OR combination.
 34. Theapparatus of claim 31, wherein said processor is configured to calculatethe amount the at least one first flag signal for the digital inputsignal and the at least one second flag signal match each other as atleast one of the following: a count of matches of the at least one firstflag signal and the at least one second flag signal over a time frame; aratio of a number of matches of the at least one first flag signal andthe at least one second flag signal to a total number of either type ofthe flag signals over the time frame; and a variance of a number ofmatches of the at least one first flag signal and the at least onesecond flag signal over the time frame.
 35. The apparatus of claim 31,wherein said processor is configured to: calculate an amount the atleast one first flag signal for the digital input signal and the atleast one second flag signal match with each other; and generate aperformance check signal indicative of the amount, with the performancecheck signal comprising an alert flag issued when the amount falls belowa performance quality threshold.
 36. The apparatus of claim 31, whereinsaid processor is configured to calculate the estimate of at least oneoperating parameter of the signal processing chain as a target valuebased on a respective performance function by reducing a gradient of therespective performance function.
 37. A non-transitory computer-readablemedium to be loaded into a memory of at least one processor for causingthe processor to operate a signal processing chain to produce an analogoutput signal from a digital input signal by: generating a least onefirst flag signal for the analog output signal and at least one secondflag signal for the digital input signal, with the at least one firstand second flag signals having a first level and a second level and eachbeing set to the first level when a respective signal from which the atleast one first and second flags is generated has a value within anamplitude window; and calculating, as a function of the at least onefirst flag signal and the at least one second flag signal, at least oneof an amount at least one first flag signal for the analog output signaland at least one second flag signal for the digital input signal matcheach other, and an estimate of at least one operating parameter of thesignal processing chain.